When a bridge over a body of water requires piers, foundations are formed by lowering caissons into the riverbed and filling them with concrete. Caissons are enormous boxes or cylinders composed of wood, metal, or concrete. Towers are erected atop caissons in the case of suspension bridges. Gates or doors on each side allow traffic to pass under the bridge.
Bridges can be made of many different materials including wood, steel, and concrete. The type of material used to build a bridge depends on what kind of load it has to carry and how much money you want to spend. A wooden bridge is cheapest but not very durable. A steel bridge is more resistant to damage from vehicles but costs more than a wooden one. A concrete bridge is most durable but also most expensive.
The key to a strong bridge is having an even distribution of weight across its total length. This means that any one point on the bridge must bear some of the entire weight of all the vehicles crossing it. If this point becomes too heavy, then it will sink down toward the riverbed beneath it. This could cause other parts of the bridge to collapse as well.
The best way to protect against this problem is by using pier supports. These can be pillars or large rocks that stick up out of the water. They distribute the weight of the vehicles across a large area rather than concentrating it onto a small part of the bridge.
Various technologies are used to construct bridges across water (depending on the level of water and quality of soil). The first option is utilized for bridges built in shallow water. The foundations of a bridge are put in low-depth water by temporarily filling a specific site, over which piers are built (a type of pillar can be built). These piers hold up the deck of the bridge. As long as there is no movement on the riverbed, the structure will not fail.
If the water is deep enough, then gravity can be used instead. For example, for small structures that will not require heavy vehicles to pass over, a platform or scaffolding can be set up at the right depth and secured into the ground. Heavy materials can then be brought up to the surface and placed onto the platform/scaffolding to create the foundation for the bridge.
Finally, if the water is very deep and requires a load-bearing structure, then floating bridges are constructed. They use large buoyant objects held aloft by cables attached to towers built on land.
For example, one of the most famous floating bridges in the world is the Golden Gate Bridge in San Francisco. It uses large concrete pontoons that connect with steel trusses forming a rigid framework. On top of this sits another layer of concrete, creating an arch bridge that can carry heavy vehicles.
Floating bridges are constructed from enormous, water-tight concrete pontoons that are rigidly attached end-to-end and upon which the highway is erected. Despite their massive concrete construction, the weight of the water displaced by the pontoons is equivalent to the structure's weight (including all traffic), allowing the bridge to float. The highway then rests on top of the pontoons.
The first floating bridges were built in the 1920s by the International Bridge Company. Since then, many other companies have built similar bridges across the world. Even today, many countries with no tradition of heavy vehicle traffic use these bridges because they are cheaper than building conventional bridges.
These bridges require regular maintenance to ensure their safety for vehicles and people. Any holes or cracks in the concrete need to be filled or the pontoon may leak, causing roadways below to become submerged. Also, the connection between the pontoons must be tight or else the bridge will break down over time due to gravity pulling them apart.
Over the years, floating bridges have been used for various applications where the ability to float is beneficial. For example, a floating bridge can cross very large bodies of water without disrupting the aquatic life underneath it. This type of bridge is called an "aquatic center span" bridge because its main purpose is to connect two regions of land together rather than carry traffic.
There are also floating bridges that combine road and rail transport modes.
When people first began to create bridges, they used simple materials such as chopped hardwood logs or planks and stones, with a simple support and crossbeam arrangement, and sometimes natural fibers braided together to retain things. As time went on, people began to create more sophisticated bridges using metals for the support beams and panels. Today, most modern bridges are built using this approach.
The first true bridge is said to have been built by Thebes (in present-day Greece) around 6500 B.C., although it may have been done as early as 8000 B.C. It consisted of a single beam supported on pylons. The ancient Egyptians called this type of bridge "shadouf" and used them in all kinds of weather and terrain. They also invented a way to make Shadoufs even stronger by bridging two beams together.
In Europe, people started building bridges about 500 A.D. But because there were no metals available at that time, they used wood instead. The Romans were the first to use stone as a supporting material for their bridges. They also used brick as a retaining wall on both sides of the river opposite the bridge structure.
During the 11th century, Europeans began using metal for their bridges. Because of this new development, many traditional wooden bridges were destroyed. However, some preserved examples can be found in European history books.
The bridge's "legs" that support the piers and piling cap or footing under the water or ground line; a deep foundation unit implanted in the earth by putting new concrete in a drilled hole with steel reinforcing. The capacity of drilled shafts is determined by the surrounding soil and/or rock. If soft, dig down twice the diameter of the shaft; if hard, dig down only once the diameter of the shaft.
The leg must be long enough to reach below the frost line and must be driven far enough into the ground so that it will not be affected by seasonal freezing and thawing. If the top of the leg freezes, it won't affect the bearing pressure on the pylon any more than if it was covered with snow. But if the bottom of the leg freezes, it could cause the center of gravity of the structure to shift and possibly damage it.
Freezing and thawing of the soil under a bridge may also have an effect on the structural integrity of the bridge. As the temperature changes, the amount of water in the soil expands and contracts, which can cause the soil to dry out or flood, which can lead to erosion.
If a drilled shaft becomes saturated with water, it can cause the bridge to collapse. The weight of the traffic on the bridge can force even more water into the shaft, causing the bridge to sink further into the mud.